1. Field of the Invention
The present invention relates to pilot operated hydraulic control valves; and more particularly to electrically operated pilot valves with a position feedback mechanism.
2. Description of the Related Art
Agricultural tractors and other types of hydraulically operated machinery commonly have components that are moved by a hydraulic cylinder/piston arrangement. The piston slides within the cylinder and divides the cylinder interior into two chambers. By selectively applying hydraulic fluid under pressure to one chamber and draining hydraulic fluid from the other chamber, the piston can be forced to move in opposite directions within the cylinder. Such movement drives a rod connected between the piston and a component of the machinery.
With reference to FIG. 1, a basic hydraulic system 10 for a machine comprises the cylinder 12 and piston 14 which is connected by a manually operated spool control valve 16 to a pair of return and supply lines 18 and 20. The supply line 20 receives pressurized fluid from a pump 22, while the return line 18 carries hydraulic fluid from the cylinder 12 back to a tank 24. The control valve 16 is a conventional manually operated, three-position, four-way spool valve with a pair of workports 15 to which the chambers of the cylinder 12 connect. The center, or neutral, position of the control valve disconnects the hydraulic cylinder 12 from both the return and supply return lines 18 and 20. In the other two positions of the control valve 16, the supply line 20 is coupled to one of the cylinder chambers 26 or 28 and the other chamber is connected to the tank 24 via the return line 18.
There is a present trend in agricultural equipment away from manual operation of the hydraulic valves toward electrically operated valves. This not only permits the valves to be located remotely from the operator position, but also enables computer control of the valves which allows more sophisticated functions to be provided. With electrical controls, the operator manipulates a joystick or other type of electrical input device to send signals to a microcomputer based controller, thereby indicating the desired movement of the associated components on the agricultural equipment. The controller interprets the electrical signals from the operator's input device and generates control signals which operate the hydraulic valves that control a hydraulic actuator which produces the desired motion.
In order to move a conventional spool valve in reciprocal directions, solenoid operators typically are attached to opposite ends of the spool. Each solenoid is energized independently to move the spool in the appropriate direction to a position where the proper fluid flow occurs to and from the hydraulic cylinder. Although there is a relationship between the magnitude of electrical current applied to a solenoid and the resultant position of spool, that relationship varies from valve to valve and also changes during the life of each valve due to a number of factors. Therefore, various types of position sensing devices have been attached to the spool valve provide an electrical feedback signal to the controller indicating the actual position of the spool. The controller compares the actual position to the desired position of the spool and adjusts the electric current applied to the solenoid coil to place the spool at the desired position. Although such position sensing feedback mechanisms operated satisfactorily, they required additional electrical components, thus adding to the expense and complexity of the solenoid operated spool valve.
In addition, there is a limit to the force and stroke that a solenoid actuator can apply to the spool control valve, which in turn limits the flow and pressure capability of the valve. To overcome these limitations, the spool control valve can be operated by a pilot valve which is directly controlled by the solenoid actuator. Although a pilot valve operator achieves higher flow and pressure capability from the main control valve, it too has drawbacks in performance, such as hysteresis, position resolution, and the ability to respond to small changes in the commanded position. These limitations result from the open loop nature of pilot operated valve control. Thus, a better control mechanisms are desired for electrically operated spool valves.